Leather is manufactured and finished on a commercial scale in tanneries. There are several different methods to tan leather. Two main variants are vegetable and chrome tan. Leather is generally colored by dyeing. Various finishes can be applied such as embossing or buffing. Those methods attempt to give some pattern, texture and depth to the surface, but have limited impact.
Leather printing can be difficult since ink generally does not bond well to a non-uniform, organic, complex substrate such as leather. Further, printing on leather is difficult due to non-uniformity of surface properties, e.g., shape, color, density, content (e.g. water, proteins, fats, oils and/or lacquer), pore concentration, flatness, thickness and surface roughness. Different regions of a hide may have different such properties due to differences in exposure to the sun. Thus, there is likely to be significant variation on a single hide as well as between hides of a batch. Techniques for printing on leather may suppress at least one usual property of leather, e.g., flexibility, surface roughness, appearance, feel (to human touch), breathability and/or absorption. One of the desirable characteristics of leather is flexibility. It is often used in places where it is required to bend and flex (e.g., footwear, seating, accessories, etc.). Printing onto the surface of leather is disadvantageous if the ink is weakly bonded and thus easily removed during normal wear and tear, or if the print cracks when flexed.
Additional difficulties may be encountered in printing on leather using printing techniques known in other technical fields involving printing on non-leather substrates if the ink molecules do not bind well with leather. For example, printing onto leather using printing techniques more commonly applied to paper may result in only some ink molecules binding to the leather so that the printed coloring/pattern is uneven and/or faint. Such coloring/patterning merely printed onto a leather surface may wear off easily, quickly and/or unevenly and/or have low light fastness and, thus, may not be durable.
There remains a need for a method of coloring or patterning leather by printing in a manner that minimizes impairment of at least one property of leather and preferably has advantages such as, inter alia, durability, high definition coloring/patterning, good resistance to heat/strain induced damage (e.g. reduced cracking), low transfer of color in wet conditions (i.e. low running of color dye/ink when wet), low cost and/or efficient (e.g. faster). Given the volumes of leather produced and finished each year, there is also a need for a fit-for-purpose industrial leather printing process operable in the harsh environment of a tannery or finishing plant.
The following disclosures are referred to:
GB2379188A;
WO/2006/129604; and
WO/01/32434,
for additional background.